1. Field of the Invention
The present invention relates to an ink tank for supplying ink to an ink-jet recording apparatus.
2. Description of the Related Art
Ink-jet recording apparatuses which form images by discharging ink onto a recording medium require an ink tank for storing the ink to be discharged. Various conventional arrangements have been proposed for ink tanks. One example, disclosed in Japanese Patent Laid-Open No. 5-8404, is a proposal for an ink tank wherein ventilation holes are formed at each corner of the ink tank so as to communicate with the atmosphere and wherein at least the inner face of the ventilation holes is formed of a water-repellant material, so that regardless of the attitude for fixing the ink tank, one of the ventilation holes is always situated above the ink level such that the inside of the ink tank communicates with the atmosphere, with the ink being supported by the surface tension of the ink at the ventilation holes lower than the ink level, thereby preventing ink from leaking out from the ventilation holes even in the event that the air in the ink tank expands due to a rise in ambient temperature. Such an ink tank design allows ink to be filled to the fullest capacity of a non-deforming hard case instead of a conventionally-proposed ink tank using a flexible pouch, and the ink storage capacity efficiency is high since all of the stored ink can be used up by introducing external air while using. This realizes a large-capacity ink tank which uses little space.
There are a great many such proposals, such as Japanese Patent Laid-Open No. 5-162325, Japanese Patent Laid-Open No. 8-118676, Japanese Patent Laid-Open No. 8-192519, Japanese Patent Laid-Open No. 9-141891, and so forth.
All of the ink tanks in the aforementioned proposals use an air/fluid separating member having air pores with water repellence. This member is formed of a film material of stretched PTFE (poly tetra fluoro ethylene), which has small pores and suitable water repellence. Accordingly, even in the event that fluid comes into contact with the air pores on the surface thereof, the fluid does not pass through the film due to its own surface tension, while gases pass through the film.
Now, it is a prerequisite of the conventional art that the air/fluid separating member and the air within the ink tank be in contact at some portion with the atmosphere at all times, regardless of the attitude of the ink tank. However, none of the aforementioned proposals can be the to truly disclose specific methods for satisfying such conditions. For example, in the event that there is a little air within the ink tank, the air is trapped at the upper portion of the ink tank, and in the event that the inner face of the ceiling of the ink tank is horizontal, the body of air may become stationary at an arbitrary position on the inner face of the ceiling of the ink tank. In the event that the ink is in contact with the entire surface of the air/fluid separating members provided at the corners of the ink tank, and in the event that the ambient temperature rises and the air which is stationary at the arbitrary position on the inner face of the ceiling of the ink tank expands, which raises the internal pressure of the ink tank, and this pressure exceeds the withstanding force of the air/fluid separating members, ink will permeate the air/fluid separating members and further leak out from the ink tank. Under such conditions, not only does the air/fluid separating member not function normally, but also invites ink leakage.
Of course, with an arrangement wherein the air/fluid separating member is provided on all wall faces, air would come into contact with the air/fluid separating member regardless of the stage, and the functions thereof would be fulfilled. However, the air/fluid separating member is relatively costly, so the area of air/fluid separating member used is preferably as small as possible, in order to reduce manufacturing costs of the ink tank.
Also, there are various types of ink stored in the ink tank, with components which inhibit the water-repellent capabilities of the air/fluid separating member in order to achieve a wide range of objects, such as improving the permeability of the ink on the recording medium upon which images are formed. For example, in the event that a surface-active agent is contained in the ink, the ink more readily adheres to the air/fluid separating member as compared with arrangements otherwise. Particularly, in the event that the air/fluid separating member is provided to the ceiling portion of the ink tank, there have been cases wherein ink remains adhered to the entire surface of the air/fluid separating member. In this case as well, the surface of the air/fluid separating member is closed off by the ink, so the air/fluid separating member can no longer transmit gas and accordingly looses its function.